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Background Liposomes are used to carry pharmaceutical agents and to alter the lipid composition of cell membranes. This study compared resonance energy transfer (RET), fluorescence dequenching, and flow cytometry as monitors and quantifiers of fusion between liposomes and mammalian spermatozoa. Methods Preliminary experiments used RET to determine the optimum sperm concentration for fusion of DL‐α‐phosphatidylcholine dipalmitoyl (PC)/DL‐α‐phosphatidylethanolamine dipalmitoyl (PE) liposomes at 35°C ± 5 mM Ca2+. Microscopy confirmed the fusion of liposomes, not just adhesion (n = 3). Dequenching tested the time‐dependent fusion of liposomes of two different lipid compositions to sperm, both, (n = 3) ± 1 mM Ca2+ and (n = 3) without Ca2+ at two sperm concentrations. Finally, flow cytometry absolutely quantified the percentage of sperm fusing to liposomes at different liposome‐to‐sperm ratios (n = 4) and with sperm from different donors (n = 3). Results RET detected fusion of liposomes with sperm and microscopy confirmed the interaction to be true fusion. Dequenching detected more fusion of liposomes with sperm at 100 × 106 sperm per milliliter than at lower concentrations (P < 0.05). Fusion dynamics differed with lipid composition but Ca2+ had no effect. Flow cytometry reliably quantified the percentage of sperm fusing with liposomes, which varied from bull to bull (P < 0.05). Conclusion Liposome fusion with mammalian sperm membranes can be quantified cytometrically and varies with lipid composition, sperm‐to‐liposome ratio, and individual animals. Cytometry 49:22–27, 2002. © 2002 Wiley‐Liss, Inc.